1. Field of the Invention
The present invention relates to a method for controlling a valve in a hydraulic brake system.
2. Description of Related Art
Modern brake systems, which are designed for a vehicle dynamics control system, normally have multiple valves, which may be used for switching between a foot-operated braking operation and an automatic braking operation.
FIG. 1 shows the essential part for the present invention of a hydraulic brake system, which is designed for implementing a vehicle dynamics control system. The part of the brake system shown includes a brake master cylinder 1, a switchover valve (USV) 2, which is normally open, a high-pressure selector valve (HSV) 3, which is normally closed, a hydraulic pump (HP) 4, and a wheel brake 5 situated on the wheel. HSV 3 is normally designed as a two-stage valve; however, it may also be designed as a single-stage valve.
The brake line exiting brake master cylinder 1 branches to USV 2 and to HSV 3. The corresponding lines are denoted by reference numerals 6 and 7. Hydraulic pump 4, which is situated downstream from HSV 3, is able to transport brake fluid from brake master cylinder 1 to wheel brakes 5 when USV 2 is open, and thus automatically build up the brake pressure in the wheel brakes.
In the case of a normal braking process in which the brake slip on the wheels is low, i.e., the vehicle dynamics regulator is not active, a specific brake pressure builds up depending on the intensity of the foot pedal actuation on brake master cylinder 1. This brake pressure is relayed to wheel brake 5 via open USV 2 along the path denoted by arrows a. HSV 3 is closed in this case. The vehicle is then decelerated in accordance with the driver input.
As soon as the wheel slip exceeds a specific threshold in a braking or acceleration maneuver, a vehicle dynamics regulator or another driver assistance system, such as ACC or TCS, is controlled and automatically increases the brake pressure acting on brake 5. In this case, a regulator 8 determines a setpoint brake pressure which is to act on wheel brake 5. This brake pressure is normally set by a corresponding input of a setpoint speed for hydraulic pump 4. During the pressure regulation, USV 2 is closed and HSV 3 is open. Hydraulic pump 4 then delivers brake fluid along the path denoted by arrows b from brake master cylinder 1 via brake line 7 to the wheel brake, thus building up the necessary pressure.
To ensure that hydraulic pump 4 is able to obtain a sufficient amount of brake fluid from the brake fluid reservoir, it must be ensured that HSV 3 opens quickly and widely enough. From the related art, it is known to actuate HSV 3 with the aid of an electrical signal having various phases, as shown in FIGS. 2 and 3.
FIGS. 2 and 3 show typical curves of control current IHSV for an HSV 3. In this connection, FIG. 2 shows the current curve for a two-stage HSV 3 and FIG. 3 shows the current curve for a single-stage HSV 3.
In the case of a two-stage HSV 3 (FIG. 2), the application of a current IV initially opens a valve pilot stage which releases a small flow cross section. This causes the differential pressure prevailing on valve 3 to be reduced slowly. Since the differential pressure applied to HSV 3 has the effect of closing HSV 3, this makes it easier to open the main stage in a second step. After the end of a predefined time span tV, a higher current IR is then applied for a short period of time tR (refresh pulse 9) in order to completely open the main stage of the valve. This is followed by a holding phase of duration tH having a lower current IH, which is set in such a way that valve 3 remains open. To ensure that the valve remains open in any case, refresh pulses 9 are applied regularly. Refresh pulses 9 are each in turn followed by a holding phase tH having a lower current value IH.
In the case of a single-stage HSV 3 (FIG. 3), the pilot stage is omitted, i.e., the process begins by applying a pulse 9, followed by a holding phase tH having a current IH. This is followed periodically by additional pulses 9 and holding phases tH.
The control methods known from the related art have in common that length tV of the control of the pilot stage, as well as the lengths of holding phases tH, are fixedly predefined. This results in the problem that the main stage is always opened by a refresh pulse 9 after a fixedly predefined period of time tV, even if the situation actually allowed a longer control of the pilot stage. If the valve is opened, the problem results that a refresh pulse 9 is always generated after a predefined holding time tH, even if holding time tH could be longer in the relevant situation. The result of this is that the valve switches unnecessarily often in many situations, causing the noise load to be relatively high. Furthermore, an excessively high electrical power usually flows through the coil of the HSV, making it possible for the valve to be heated to an unnecessarily high degree and overheat in the extreme case for example, long duration of a braking maneuver).